Ingenious mechanics and progression. Minutes turn to hours very quickly, "ah I just need to redo my copper smelting before I go, but how is that going to impact my ore dropoff rail station, and then shouldn't I redo my track switches so my trains get in faster? What about..." etc etc. As addictive in that sense as Civ in its best moments.
Dev team is engaged, thorough, perfectionist yet realistic. Very high self standards, very focused on optimizations. They have created a masterpiece and are lovingly refining it. I have gotten my money's (US$20) worth out of this and then some!